As is known, chemical sensors detect the presence of gas on the basis of a chemical reaction which takes place between the molecules of the gas and a sensitive film. The chemical reaction depends significantly on the operating temperature which influences the effects of adsorption, desorption and diffusion of the gas in the film. Consequently, temperature is an important factor in optimizing the performance of the sensor, particularly as regards sensitivity, selectivity and response time. To guarantee optimum operation, therefore, the sensors are provided with means for regulating and controlling temperature.
Recently, integrated chemoresistive gas microsensors, the manufacture of which makes use of microelectronics techniques, have been proposed and produced. These microsensors have the following advantages: reduced manufacturing costs, low energy consumption in operation, high response times and integrability with the temperature control and output signal processing circuit.
Integrated gas microsensors using chemoresistive membranes based on tin oxide are appearing on the market; on the surface of such membranes, deposited on a wafer of semiconductor material machined using the technique of "bulk micromachining," described below, a chemical reaction takes place between the oxygen of the membrane and the gas to be detected which has the effect of changing the resistance of the film and thus enables the presence of the gas to be detected.
In order to operate correctly, such sensors must be maintained at temperatures of approx. 400.degree. C., so they are provided with heater elements and must be thermally insulated from the rest of the chip, which includes the integrated signal processing and control circuit.
Various techniques for isolating the sensitive part from the rest of the chip are known in literature. The technique used historically consists of "bulk micromachining," comprising producing the sensitive part on top of or inside a dielectric layer deposited on a solid silicon wafer and removing a portion of solid silicon from the back of the wafer with wet etching methods. The dielectric layer performs the dual task of mechanically supporting the sensor and thermally insulating the sensor from the wafer of solid silicon. In the context of this technique prototypes have been produced with partial removal of the silicon from the area of the sensor, in which the excavation is carried out only on part of the thickness of the wafer, and prototypes which provide the total removal of the silicon at the area of the sensor (the etching reaches as far as the dielectric layer carrying the sensor element). As regards this second solution, reference may be made for example to the article entitled "Basic Micro-Module for chemical sensors with on chip heater and buried sensor structure" by D. Mutschall, C. Scheibe, E. Obermeier.
On the other hand the technique of bulk micromachining requires the presence of front-back machining processes and comprises particular demands for handling the chips which are such that it proves to be incompatible with current integrated circuit manufacturing methods.
Another proposed technique consists of "front micromachining" on the basis of which the wafer of solid silicon is etched from the front and a dielectric layer mechanically supports and thermally insulates the sensor element. In this respect, for the production of a different type of sensor, reference may be made for example to the article by D. Moser and H. Baltes entitled "A high sensitivity CMOS gas flow sensor based on an N-poly/P-poly thermopile," DSC-Vol. 40, Micromechanical Systems, ASME, 1992; furthermore, for a survey of the techniques of bulk and front micromachining, reference may also be made to the article entitled "Micromachining and ASIC technology" by Axel M. Stoffel in Microelectronics Journal, 25 (1994), pages 145-156.
This technique for producing suspended structures does, however, require the use of etching phases that are not very compatible with the current manufacturing processes used in microelectronics and does not therefore permit sensors and the related control and processing circuitry to be obtained on a single chip.
Furthermore, the use of dedicated SOI (Silicon On Insulator) substrates has been proposed, in which the starting wafer comprises a stack of silicon/silicon oxide/silicon, with the oxide selectively removed at the sensor area, forming an air gap. The excavations made from the front of the wafer at the end of the process phases to contact the air gap enable the sensor to be thermally insulated. In this respect, for a shear stress sensor, reference may be made for example to the article by J. Shajii, Kay-Yip Ng and M. A. Schmidt entitled "A Microfabricated Floating-Element Shear Stress Sensor Using Wafer-Bonding Technology," Journal of microelectromechanical systems, Vol. 1, No. 2, June 1992, pages 89-94. The method used for bonding (apart from the formation of the air gap) is further described in the article "Silicon-on-Insulator Wafer Bonding-Wafer Thinning Technological Evaluations" by J. Hausman, G. A. Spierings, U. K. P. Bierman and J. A. Pals, Japanese Journal of Applied Physics, Vol. 28, No. 8, August 1989, pages 1426-1443. Finally, the use of a dedicated SOI substrate is also described in European patent application No. 96830436.0 filed on 31.7.96 in the name of this applicant.